This invention relates to means for minimizing arcing within a cathode ray tube and more particularly to improved means for utilization in a cathode ray tube to inhibit the buildup of surface charge in the neck region thereof.
With the advance of cathode ray tube technology, there has been a trend toward miniaturization and compaction of electron gun structures, which in turn are encompassed within envelope neck portions of smaller diameters and shorter lengths. Consequently, spacings between the electrode components of the electron gun structure and the adjacent sidewall of the enclosing neck portion have become increasingly smaller. This condition is particularly true in color cathode ray tubes of in-line gun construction wherein the side-oriented guns in the gun assembly are particularly close to the wall of the neck.
During cathode ray tube manufacturing, especially in the tube exhaust and processing steps, the cathode is operated or conditioned at a much higher temperature than that subsequently encountered during normal operation. As a result, there is an effusion or metallic vaporization, particularly of nickel, which emanates from the cathode and thence deposits on adjacent surfaces. Since the wall of the neck is particularly close to certain of the electron guns in the gun assembly, a conductive metallic sublimation is naturally deposited thereon.
In cathode ray tube construction it is conventional practice for the funnel-disposed conductive coating to extend into the forward region of the neck portion, such coating usually being of the high positive electrical potential of the final electrode of the gun assembly. The adjacency of this high potential conductive coating at the forward end of the neck portion, in conjunction with the neighboring cathode sublimation deposition at the rear of the neck portion, creates an arc-fostering condition during tube operation. This neck-oriented electrical conductive sublimation deposition is conducive for the relatively slow build-up of a deleterious negative charge effect on the interior surface of the neck, which upon reaching a breakdown level discharges as an areal release of current toward the forwardly disposed positively charged conductive coating with the effect of a soft arc. This type of arcing is a phenomenon somewhat analogous to the charge and discharge of a capacitor whereof the discharge is normally non-destructive. Such arcing is normally repetitive with the gradual buildup time being in the order of several seconds to a plurality of minutes. The resultant discharge is audibly and visibly annoying to the observer since it affects both the audio and video portions of the circuitry associated with the operating tube. The audio effect is usually apparent as a "click", whereas the video influence is normally evidenced in two ways, first in the form of a momentary raster disturbance such as an areal constriction or distracting flicker, and, secondly, as an abrupt change of edge color-definition and sharpness of the imagery in the screen display, which is commonly referred to as static mis-convergence. For example, in a shadowmask type of color cathode ray tube employing an in-line gun assembly, the electron beams on either side of the gun structure are indirectly influenced by the surface charge buildup on the rearward portion of the neck. As the negative charge increases, there is also evidence of a counter positive charge build-up emanating from the extension of the funnel coating onto the adjacent glass at the forward end of the neck. In some instances, this charge effect is further promoted by minute depositions of surface contamination emanated from the adjacent gun electrodes during tube conditioning. As the positive charge migrates beyond the coated area per se, further into the neck toward the negatively charged area, it tends to effect an under-converging influence on the beams at the forward end of the gun assembly, especially through the gap between the G3-G4 electrodes. The presence of this positive charge influence gradually causes the outer beams to move slightly outward from their normal paths. Such change in beam positioning is noticeable as a gradual deterioration in the color imagery of the screen display wherein the edge definitions of display elements become gradually multicolored and lose sharpness due to misplacement of beam impingement on the screen. This condition, referenced as static convergence drift, increases until arc discharge, whereupon the beams revert to their normal positioning causing an abrupt and noticeable change in edge-sharpness. Consequently, the edge-definition again gradually deteriorates as the charge build-up relationship repeats.